CN115326873A - Circular tube surface natural convection heat transfer coefficient test analysis and evaluation method based on DBD discharge device - Google Patents
Circular tube surface natural convection heat transfer coefficient test analysis and evaluation method based on DBD discharge device Download PDFInfo
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Abstract
The invention discloses a DBD discharge device-based method for testing, analyzing and evaluating natural convection heat transfer coefficients of the surface of a circular tube, wherein a DBD reactor is used as a heat source in the circular tube, a calorimetry method is adopted for measuring the thermal power of the DBD discharge device for heating gas in the circular tube, and the temperature distribution of different angular positions of the wall surface is measured by a method of uniformly arranging a plurality of temperature measuring points on the outer wall surface of the circular tube; and then the coupling effect of the radiation heat exchange and the natural convection heat exchange of the wall surface of the circular tube and the change of thermodynamic parameters caused by the change of the air temperature are considered, the natural convection heat exchange coefficient of the wall surface of the circular tube is obtained, and the Knudsen number empirical formula of the wall surface of the circular tube when the Rayleigh number (Ra) value is within the range of 1.8 multiplied by 105 and less than or equal to 3.5 multiplied by 105 is obtained through data fitting, so that the problems of non-concentric installation of the electrodes of the heater and non-uniform heating of the wall surface of the cylinder caused by the air gap between the electrodes and the wall surface during the experimental measurement of the natural convection heat exchange coefficient of the outer wall surface of the traditional pipeline are effectively solved.
Description
Technical Field
The invention belongs to the technical field of test and analysis of a natural convection heat transfer coefficient of a circular tube surface, and relates to a test, analysis and evaluation method of the natural convection heat transfer coefficient of the circular tube surface based on a DBD discharge device.
Background
The natural convection heat transfer of the heated cylinder wall surface has wide application in engineering, such as heat exchangers, solar water heaters, power plant boiler pipe cooling, heating ventilation air conditioning systems, computer cooling, nuclear reactor cooling and the like. This makes the convective heat transfer characteristics of the cylindrical wall always a focus of the researchers. Therefore, the method for accurately measuring the natural convection heat transfer parameters on the surface of the cylinder has important significance for basic physical research and heat transfer control under different application occasions.
At present, to the temperature distribution and the natural convection heat transfer characteristic of cylinder wall, the researcher mainly adopts the experimental research mode to carry out the analysis, because the heating mode of drum is the crucial factor that influences the convection heat transfer parameter measurement accuracy, but generally adopts the mode of installing electric heater in the drum or utilizing direct current power supply direct heating drum as the heat source in current experimental research, at the in-process of realizing this application, the inventor finds that current heating mode has following problem at least:
1. in order to provide a stable heat flow, the electric heater needs to be installed at the central axis position of the cylinder. Therefore, the concentricity problem of the heater installation can cause the cylinder to be heated unevenly, thereby influencing the measurement results of experimental parameters such as the flow and the temperature of the wall surface;
2. because the installation position of a heat source in a cylinder has obvious influence on the convective heat transfer of a wall surface, the heat source deviates from the axis of the cylinder to cause the deflection of plumes around the cylinder, so that the heat transfer presents obvious asymmetry, and in order to solve the problem of uneven distribution of heat flow on the wall surface of the cylinder, in the prior art, various materials with higher heat conductivity coefficient are filled in gaps between an electric heater and the wall surface of the cylinder to increase the heat conductivity between the cylinder and the heater, but even after the materials with higher heat conductivity coefficient are filled in the gaps, the temperature distribution at the same circumferential position of the wall surface of the cylinder still has larger unevenness, and the influence can be avoided only by installing a plurality of thermocouples at the same circumferential position of the cylinder and then obtaining the average temperature;
3. at present, electric power of a heater is generally adopted as thermal power of a heating cylinder in an experiment, but the thermal efficiency of the electric heater cannot reach 100%, so that deviation exists between measured heat flow and actual heat flow, and the accuracy of an experiment result is further influenced.
Disclosure of Invention
In order to solve a series of problems caused by the measurement accuracy of the electric heater on the natural convection heat transfer of the wall surface of the cylinder, the invention adopts a DBD reactor as a heat source in the cylinder and provides a test, analysis and evaluation method for the natural convection heat transfer coefficient of the surface of the circular tube based on a DBD discharge device.
The purpose of the invention can be realized by the following technical scheme:
the invention provides a DBD discharge device-based method for testing, analyzing and evaluating natural convection heat transfer coefficients of the surface of a circular tube, which comprises the following steps:
s1: a calorimetry method is adopted to measure the thermal power transferred from the DBD reactor to gas flow, and the method specifically comprises the following steps:
s11: the volume flow O of air at the inlet is adjusted to 200L/min by adjusting the power-on frequency of the fan and the gas control valve]And calculating the airflow density rho according to the airflow temperature at the inlet of the cylinder in And then through the formula of calculation of the mass flow rate of the air flow
Obtaining aeration flow mass flow rate
S12: when the discharge of the reactor reaches the stable state, the temperature at the inlet and the outlet of the reactor is basically kept unchanged, and the thermal power P of the DBD reactor is obtained by calculating through a thermal power calculation formula of the DBD reactor a ；
S2: analyzing the average convective heat transfer coefficient, the radiant heat transfer coefficient and the Knudel number of the cylinder wall surface by adopting the average temperature of all temperature measuring points of the cylinder wall surface;
s3: analyzing uncertainty;
s4: testing results and analyzing;
s5: and drawing a conclusion.
In one possible embodiment, the DBD reactor thermal power is calculated by the formula
In the formula, C p,in And C p,out The constant pressure specific heat, T, of the air at the inlet and outlet of the reactor in And T out The temperatures of the gas streams at the inlet and outlet of the reactor, respectively.
In a possible implementation manner, the specific implementation method corresponding to the step S2 is as follows:
s21: setting 8 angular positions on the cylindrical wall surface, distributing 7 temperature measuring points in an axial distribution area at any angular position of the cylindrical wall surface, and collecting the temperature of each temperature measuring point;
s22: the temperature of each angular position of the wall surface of the cylinder is carried out corresponding to each temperature measuring pointCarrying out average value processing to obtain the average temperature of any angular position theta of the wall surface of the cylinder
T θ,i The temperature of the ith temperature measuring point is expressed as any angular position theta of the wall surface of the cylinder, wherein i is expressed as the number of the temperature measuring points, i =1,2, · 7;
s23: calculating the average temperature of the cylindrical wall surface based on the average temperature corresponding to each angular position of the cylindrical wall surface
The expression is
S24: detecting external ambient temperature T air And is based on
And T air Calculating the radiant heat exchange quantity Q in unit time r Wherein
Wherein ε represents the blackness of the cylinder wall surface, C 0 Is the absolute black body emissivity coefficient, d out Expressed as the outer diameter of the cylinder, L 0 Expressed as the length of the cylinder, and pi as the circumference ratio;
s25: calculating the radiation heat exchange coefficient of the cylinder wall surface, wherein the expression is
In the formula
S26: calculating the convective heat transfer coefficient h of the cylinder wall surface ec The expression is
In the formula Q c Expressed as the cylinder wall surface per unit timeConvection heat exchange amount of, and Q c ＝Q-Q r Q represents the total heat exchange amount between the outer wall surface of the inner cylinder and the environment per unit time, and Q = P a ；
S27: calculating the average Nu of the cylindrical wall surface, wherein the expression is Nu = h ec d 0 Lambda, wherein lambda is expressed as the thermal conductivity;
s28: calculating the Grataffet number Gr, the Prandtl number Pr and the Rayleigh number Ra of the air at the wall surface of the cylinder, wherein Gr = g alpha delta Td out 3 /v 2 ，
Ra = Gr & Pr, where g is gravitational acceleration, ν is kinematic viscosity, α is expansion coefficient, and C p Expressed as specific heat and μ as viscosity coefficient.
In a possible implementation manner, the specific implementation method corresponding to the step S3 is as follows:
s31: analyzing mass flow rate of air
Uncertainty of
The expression is
Wherein δ T in Expressed as the temperature uncertainty at the reactor inlet, δ O as the volumetric flow uncertainty of the air at the inlet;
s32: analysing the uncertainty δ P of the thermal power Pa of the discharge a The expression is
Wherein δ C p,in Expressed as the uncertainty of the specific heat at constant pressure of the air at the inlet of the reactor, deltaC p,out Expressed as the uncertainty of the specific heat at constant pressure of the air at the outlet of the reactor, deltaT out Expressed as the temperature at the outlet of the reactorDegree uncertainty, a, b, c, d are constants;
s33: analyzing uncertainty delta h of radiation heat transfer coefficient of cylinder wall surface er The expression is
Wherein δ Q r Expressed as the uncertainty of the radiant heat exchange per unit time, and
δd out expressed as the uncertainty of the outer diameter of the cylinder, δ L 0 Expressed as an uncertainty in the length of the cylinder,
expressed as the uncertainty of the mean temperature of the cylinder wall, δ T air Expressed as the uncertainty of the external ambient temperature;
s34: analyzing uncertainty delta h of convective heat transfer coefficient of cylinder wall ec Of the formula
Wherein δ Q c Expressed as the uncertainty of the convective heat transfer quantity per unit time, an
δ Q represents the uncertainty of the total heat exchange amount between the outer wall surface of the inner cylinder and the environment in unit time;
s35: analyzing uncertainty delta Nu of average Knudel number of cylinder wall surface, its expression is
δ λ represents the uncertainty in thermal conductivity.
In a possible implementation manner, in the step S32
In a possible implementation manner, the step S32In
In a possible implementation manner, the step S4 specifically includes:
s41: analyzing a relationship between a thermal efficiency of the DBD reactor discharge and a discharge voltage to determine whether the DBD discharge can provide a sufficient heat source;
s42: analyzing the influence of different thermal powers on the temperature distribution uniformity of the outer wall surface of the cylinder to evaluate the feasibility of the DBD reactor as a heat source in the cylinder;
s43: and comparing and analyzing the obtained Nossel number result with the existing correlation, verifying the effectiveness of the experimental system and the experimental method, and obtaining a new Nossel number correlation in the Rayleigh number range through data fitting.
In a possible implementation, the conclusion drawn in S5 is as follows:
(1) DBD reactors are feasible as a heat source for heating gas in a cylinder;
(2) The temperature distribution of the wall surface of the cylinder shows a trend of gradually decreasing from the top to the bottom, and along with the increase of the thermal power and the temperature of the wall surface, the temperature difference between the top and the bottom of the pipe wall shows a trend of increasing, decreasing and increasing under the influence of the temperature difference between the wall surface and air and convection disturbance;
(3) Compared with the conventional mode of installing an electric heater in a cylinder, the mode of adopting the DBD reactor as a heat source can obtain more uniform tube wall temperature, and the average standard deviation of the wall surface temperatures at different angular positions is within 3K;
(4) The deviation of the Knudel number obtained through experiments and the data obtained through the existing correlation calculation is generally within a 20% deviation band, the average deviation of the Knudel number is mostly less than 10%, and the accuracy of experimental test results is verified.
Compared with the prior art, the invention has the beneficial effects that:
(1) The invention discloses a natural convection heat exchange characteristic experiment system based on the outer surface of a cylinder of a linear-cylinder DBD reactor. The method comprises the steps of considering the influence of thermal radiation and the change of air thermodynamic parameters, measuring the thermal power of the reactor by adopting a calorimetry method, analyzing the uniformity of the temperature of the cylindrical wall surface by adopting a high-precision thermocouple and arranging a plurality of temperature measuring points on the cylindrical wall surface, and calculating natural convection heat transfer parameters, thereby verifying the feasibility and the effectiveness of the DBD reactor as a heat source in a pipeline.
(2) According to the invention, the DBD reactor is used as a heat source in the cylinder, the DBD reactor has the typical characteristics of uniform discharge, large discharge space, controllable discharge power, high discharge efficiency and the like, the DBD reactor can provide stable, uniform and controllable heat flow in a large range, and the heat flow can be rapidly transmitted to the wall surface of the cylinder through the discharge channel, so that the problem of nonuniform heating of the wall surface of the cylinder caused by non-concentric installation of the electrodes and air gaps between the electrodes and the wall of the cylinder can be effectively avoided.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a schematic view of the arrangement of temperature measuring points of the cylindrical wall surface of the present invention;
FIG. 2 is a schematic view of a measurement process for determining convective heat transfer parameters on the surface of a cylinder according to the present invention;
FIG. 3 is a graph of temperature and thermal power at the drum outlet versus DBD reactor discharge voltage;
FIG. 4 is a schematic diagram showing the comparison of the radiative convective heat transfer coefficient and the natural convective heat transfer coefficient;
FIG. 5 is a graph comparing the average temperature and the standard deviation at different angular positions of the cylindrical wall surface at different powers;
FIG. 6 is a graph comparing Knudsen numbers and correlations obtained by experimental measurements;
FIG. 7 is a schematic diagram of a fitting curve and correlation of Knudel number and Rayleigh number.
Detailed Description
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
The invention provides a DBD discharge device-based method for testing, analyzing and evaluating natural convection heat transfer coefficient of the surface of a circular tube in ＝2r in =30mm, outer diameter d out ＝2r out =40mm. A coaxial cylinder type DBD plasma reactor is adopted as a heat source of a heating cylinder, and a high-voltage electrode of the DBD reactor has a diameter d h Is 3mm and has a length L 0 A 400mm stainless steel rod, which is mounted at the axial position of the cylinder. The ground electrode of the DBD reactor is a layer of dense thin stainless steel mesh that is tightly wound around the outer surface of the cylinder. Because the heat conductivity of the grounding electrode is far larger than that of the aluminum oxide ceramic tube, and the thickness of the grounding electrode is very small, the influence of the grounding electrode on the natural convection heat transfer of the outer surface of the cylinder can be ignored. The high-voltage electrode and the grounding electrode are powered by an alternating current plasma power supply (CTP-2000K alternating current plasma power supply). Discharge power f of the reactor h The voltage is fixed at 12kHz, the discharge voltage can be adjusted by an external voltage regulator, and the adjustment range of the peak-to-peak voltage Vpp is 0-25kV. The discharge power and the thermal power in the cylinder can be adjusted by varying the discharge voltage.
When the thermal power of the DBD discharge device is measured by adopting a calorimetry method, a variable frequency fan (RB-41D-A1) is connected to the inlet of the cylinder to be measured through a flexible hose. The rotating speed of the fan is adjusted through an external frequency converter, and the flow of air at the inlet of the cylinder can be effectively adjusted by assisting with a control valve at the outlet of the fan. The flow of air at the drum inlet was measured using a rotameter. And after entering the cylinder, the airflow is subjected to discharge reaction by the wire-cylinder DBD reactor, and the heated gas flows out from the cylinder outlet. The gas flow temperatures at the inlet and outlet of the cylinder were measured by mounting a low temperature thermocouple with a precision of + -0.1K at the inlet of the cylinder and a high temperature thermocouple with a precision of + -1.5K at the outlet of the cylinder, respectively. And extracting a temperature signal by adopting a multichannel temperature acquisition module with the sampling frequency of 10Hz and upper computer software so as to calculate the thermal power in the discharging process. Because partial energy in the heat energy transferred to the gas by the reactor is dissipated through natural convection heat exchange between the outer surface of the cylinder and the external environment, in order to avoid the influence of the partial energy on the thermal power test, the heat insulation treatment is carried out by tightly winding polycrystalline mullite fiber with low thermal conductivity and high temperature resistance on the outer surface of the cylinder.
When measuring the natural convection heat transfer parameters of the wall surface of the cylinder, the gas and the cylinder are directly heated by taking the thermal power discharged by the DBD as a heat source instead of introducing airflow by using a fan. And measuring the temperature of the wall surface of the cylinder by adopting a K-type thermocouple so as to obtain the temperature distribution of the wall surface and calculate the natural convection heat transfer parameters. When the mode of pasting or welding a thermocouple on the wall surface of the cylinder is adopted, the thermocouple probe and the wiring thereof influence the flow and the heat transfer of the wall surface of the cylinder. In order to solve the problem, after the DBD reactor discharges and the temperature of the cylinder wall surface is stable, 35 temperature measuring points of the cylinder wall surface are measured point by point to obtain the temperature distribution of the cylinder wall surface. In order to evaluate the influence of discharge voltage and thermal power on the temperature uniformity of the circular wall surface at different axial positions at the same circumferential position, 7 temperature measuring points are arranged at the same angle in the circumferential direction of the cylinder, and the interval L1 between the temperature measuring points is 50mm. In addition, in order to compare and analyze the temperature distribution rule of the wall surface of the cylinder at different angular positions, 8 temperature measuring points are uniformly arranged at the same axial position along the angular direction, and the temperature measuring points are spaced by 45 degrees. And after the system is stable, continuously measuring the temperature at each temperature measuring point for 10s in sequence, and taking the average value of the measured temperature within 10s as the actual temperature at the measuring point. Wherein the arrangement of the temperature measuring points of the cylinder wall is shown in figure 1. In addition, cylindrical plugs made of polycrystalline mullite fiber are arranged at two ends of the reactor, so that the influence of heat loss caused by heat transfer between two ends of the cylinder and outside air on measurement of natural convection heat transfer parameters is avoided.
S1: a calorimetry method is adopted to measure the thermal power transferred from the DBD reactor to gas flow, and the method specifically comprises the following steps:
s11: the volume flow O of air at the inlet is adjusted to 200L/min by adjusting the power-on frequency of the fan and the gas control valve]And calculating the airflow density rho according to the airflow temperature at the inlet of the cylinder in And then through the formula of calculation of the mass flow rate of the air flow
Obtaining aeration flow mass flow rate
S12: when the discharge of the reactor reaches the stable state, the temperature at the inlet and the outlet of the reactor is basically kept unchanged, and the thermal power P of the DBD reactor is obtained by calculating through a thermal power calculation formula of the DBD reactor a Wherein
In the formula, C p,in And C p,out The constant pressure specific heat, T, of the air at the inlet and outlet of the reactor in And T out The temperatures of the gas flows at the inlet and outlet of the reactor, respectively;
s2: the method comprises the following steps of analyzing convection heat transfer parameters of a cylinder wall surface by adopting average temperatures of all temperature measuring points of the cylinder wall surface, wherein the convection heat transfer parameters comprise an average convection heat transfer coefficient, a radiation heat transfer coefficient and a Knudel number, and in the whole experiment process, a measurement flow for determining the convection heat transfer parameters of the cylinder wall surface is shown in a figure 2, and the specific implementation method is as follows:
s21: setting 8 angular positions on the cylindrical wall surface, distributing 7 temperature measuring points in an axial distribution area at any angular position of the cylindrical wall surface, and collecting the temperature of each temperature measuring point;
s22: carrying out mean value processing on the temperatures of all temperature measuring points corresponding to all angular positions of the cylindrical wall surface to obtain the average temperature of any angular position theta of the cylindrical wall surface
T θ,i Expressing the temperature of the ith temperature measuring point at any angular position of the wall surface of the cylinder, wherein i is the number of the temperature measuring points, i =1,2, ·,7;
s23: calculating the average temperature of the cylindrical wall surface based on the average temperature corresponding to each angular position of the cylindrical wall surface
The expression is
S24: detecting external ambient temperature T air And is based on
And T air Calculating the radiant heat exchange quantity Q in unit time r Wherein
Wherein ε represents the blackness of the cylinder wall surface, C 0 Is the absolute black body emissivity coefficient, d out Expressed as the outer diameter of the cylinder, L 0 Expressed as the length of the cylinder, and pi as the circumference ratio;
s25: calculating the radiation heat exchange coefficient of the cylinder wall surface, wherein the expression is
In the formula
S26: calculating the convective heat transfer coefficient h of the cylinder wall surface ec The expression is
In the formula Q c Expressed as convective heat transfer per unit time of the cylindrical wall surface, and Q c ＝Q-Q r Q represents the total heat exchange amount between the outer wall surface of the inner cylinder and the environment per unit time, and Q = Pa;
s27: calculating circleThe average Nu of the cylinder wall surface is Nu = h ec d 0 Lambda, wherein lambda is expressed as the thermal conductivity;
s28: to evaluate the correlation between the Knudel number and the air physical property parameter, the Grataffet number Gr, the Prandtl number Pr and the Rayleigh number Ra of the air at the cylinder wall surface were calculated, where Gr = g α Δ Td out 3 /v 2 ，
Ra = Gr & Pr, where g is gravitational acceleration, ν is kinematic viscosity, α is expansion coefficient, and C p Expressed as specific heat, μ is expressed as viscosity coefficient;
it should be noted that the invention considers the change of thermodynamic parameter caused by the change of air temperature in the cylinder, and calculates the air density rho, viscosity coefficient mu and specific heat C in advance at different temperatures p A thermal conductivity lambda and an expansion coefficient alpha, wherein the thermodynamic parameters alpha, v, C p Mu and lambda are all in
And T air Solving under the average value of the obtained product;
s3: and (3) uncertainty analysis: the uncertainty of the experimental result is mainly related to the measurement errors of parameters such as air flow, air flow temperature, cylinder wall surface temperature and the like and the processing error of the cylinder size, and the specific implementation method is as follows:
s31: analyzing mass flow rate of air
Uncertainty of
The expression is
Wherein δ T in Expressed as the temperature uncertainty at the reactor inlet, δ O as the volumetric flow uncertainty of the air at the inlet;
s32: analyzing uncertainty deltaP of thermal power Pa of the discharge, expressed as
Wherein δ C p,in Expressed as the uncertainty of the specific heat at constant pressure of the air at the inlet of the reactor, according to an empirical formula of the specific heat at constant pressure,
δC p,out expressed as the uncertainty of the specific heat at constant pressure of the air at the outlet of the reactor,
δT out expressed as the temperature uncertainty at the reactor outlet, a, b, c, d are constants;
s33: in the measurement of natural convection heat transfer characteristics, the uncertainty delta h of the radiation heat transfer coefficient of the cylinder wall surface is analyzed er The expression is
Wherein δ Q r Expressed as the uncertainty of the radiant heat exchange per unit time, and
δd out expressed as the uncertainty of the outer diameter of the cylinder, δ L 0 Expressed as an uncertainty in the length of the cylinder,
expressed as the uncertainty of the mean temperature of the cylinder wall, δ T air Expressed as the uncertainty of the external ambient temperature;
s34: in the measurement of natural convection heat transfer characteristics, the uncertainty delta h of the convection heat transfer coefficient of the cylinder wall surface is analyzed ec The expression is
Wherein δ Q c Expressed as the uncertainty of the convective heat transfer quantity per unit time, and
δ Q represents the uncertainty of the total heat exchange between the outer wall surface of the inner cylinder and the environment per unit time;
s35: analyzing uncertainty delta Nu of average Knudel number of cylinder wall surface, its expression is
δ λ represents the uncertainty in thermal conductivity;
in a particular embodiment of the invention, the radius r of the cylinder out And length L of the cylinder 0 Uncertainty of. + -. 0.1mm, uncertainty of temperature measurement T at the reactor inlet in Is +/-0.1K, and the temperature T at the outlet of the reactor out And the temperature T of the cylinder wall θ,i The uncertainty of the measurement is ± 1.5K and the uncertainty of the flow O is 1%.
The mass flow rate under different thermal powers can be calculated by the uncertainty expression
The maximum uncertainty of the heat power Pa is 1.01 percent, and the radiation heat transfer coefficient h er The maximum uncertainty of (1) is 4.17%, and the natural convection heat transfer coefficient h ec The maximum uncertainty of (2) was 5.59%, respectively, and the maximum uncertainty of the average nussel number Nu of the cylinder wall was 5.60%. It can be seen that the uncertainty of the experimental results is within an acceptable range.
S4: the test results and analysis specifically include:
s41: analyzing a relationship between a thermal efficiency of the DBD reactor discharge and a discharge voltage to determine whether the DBD discharge can provide a sufficient heat source;
it can be understood that accurate measurement of the thermal power of the DBD reactor for heating the cylinder is the basis for calculating the wall natural convection heat transfer parameters. By adopting the calorimetry, the discharge frequency of the DBD reactor is set to be 12kHz, and the peak-to-peak voltage Vpp of DBD discharge is adjusted by an external voltage regulator, so that the value range is 14kV to 24kV, and the value interval is 0.5kV. When the DBD discharge reaction and the gas flow reach steady state, the temperature at the cylinder outlet remains substantially unchanged, the temperature at the cylinder outlet and the calculated thermal power are shown in fig. 3;
as can be seen from the figure, the temperature of the gas flow at the cylinder outlet and the thermal power of the DBD reactor for heating the gas increase with increasing peak-to-peak voltage (Vpp). When Vpp is 14kV, 18kV and 22kV, the corresponding thermal powers are 40.37W, 164.78W and 719.00W, respectively. In fact, the heating effect of the DBD reactor on the gas mainly results from collisions between particles, on one hand, inelastic collisions between high-energy electrons generated by the discharge and particles and neutral particles generate excited molecules and ionized particles to release energy; on the other hand, the ions collide with neutral particles or free electrons or quench the released energy. With the increase of the discharge voltage, the ionization reaction of the gas in the discharge gap is more severe, and more high-energy electrons will appear in the gas, so that more energy is transferred to heavy particles by the high-energy electrons through collision in the discharge process, and the thermal power of the heated gas and the temperature of the gas are continuously increased. Furthermore, it can be seen from fig. 3 that the thermal power is substantially exponential in relation to the discharge voltage.
From the above analysis, it can be known that when the frequency of the DBD reactor is constant, effective adjustment of the thermal power can be achieved by adjusting the peak-to-peak voltage Vpp of the discharge, thereby obtaining a stable heat source for heating the gas. Thus, the experimental results demonstrate the feasibility of the DBD reactor as a heat source within the cylinder.
S42: analyzing the influence of different thermal powers on the temperature distribution uniformity of the outer wall surface of the cylinder to evaluate the feasibility of the DBD reactor as a heat source in the cylinder;
in the specific embodiment, the temperature measurement and analysis experiment of the cylinder wall surface is performed based on the correspondence between the discharge voltage and the thermal power determined in S41. As can be seen from fig. 4, the temperature of the cylindrical wall surface gradually increases with the increase of the thermal power, but the trend of the increase becomes slower. In addition, the radiation heat exchange coefficient her monotonically increases with the increase of the discharge power, and the increase amplitude is large,this is because the average temperature of the outer wall surface of the cylinder is determined by the average temperature
The calculation formula shows that the radiation heat exchange quantity has a correlation with the fourth power of the temperature. In addition, the natural convection heat transfer coefficient is increased along with the increase of the thermal power when the thermal power is lower (Q is less than or equal to 99.04W), but the change is not obvious along with the continuous increase of the thermal power. When the thermal power is 40.37W, the natural convection heat transfer coefficient (hec) is 7.53W/(m 2. K), which is about 1.23 times of the radiative heat transfer coefficient (her), and the where hec value is 6.11; for a thermal power of 1471.66W, hec is 10.8698W/(m 2K), which is only 26.33% of her, where her has a value of 41.29. Thus, at higher wall temperatures, radiative heat transfer dominates, and its measurement of the natural convective heat transfer coefficient cannot be neglected.
Referring to FIG. 5, the standard deviation of the measured temperature of 7 thermocouples at each angular position and the average of the standard deviations of the temperatures at different angular positions under the same thermal power are identified by error bars
In addition, the difference value of the temperature of the bottom temperature measuring point (theta =0 ℃) and the temperature of the top temperature measuring point (theta =180 ℃) is marked in the figure, so that the difference value is used for analyzing the temperature distribution rule of the cylinder wall surface along the angular direction under different thermal powers. As can be seen from fig. 5 (a) - (d), as the thermal power increases, the temperature of the cylinder wall surface along the angular direction shows the following complex trend:
(1) When the thermal power is lower (Q is less than or equal to 99.04W), the temperature of the cylinder is gradually reduced from the top to the bottom, and the temperature difference between the top and the bottom of the cylinder is gradually increased along with the increase of the thermal power (the temperature difference is increased from 2.67K to 5.18K). The heated air floats upwards under the action of gravity, so that the temperature of the air at the top of the cylinder is higher than that at the bottom of the cylinder. When the temperature of the cylinder wall surface is lower, the floating of the hot air does not form obvious disturbance on the flow, and the heat exchange between the cylinder wall surface and the air is mainly heat conduction. The bottom of the cylinder is surrounded by unheated air, so that heat exchange is more violent, the top of the cylinder is surrounded by floating hot air, the temperature difference between the wall surface and the air is small, and the heat exchange strength is weak, so that the temperature difference between the top and the bottom of the cylinder is larger and larger;
(2) When the thermal power is 99.04W-414.33W, the temperature at the top of the cylinder is still higher than that at the bottom, but the temperature difference between the cylinder and the thermal power is gradually reduced along with the increase of the thermal power (the temperature difference is reduced from 5.18K to 3.67K). The temperature difference between the wall surface of the cylinder and ambient air is increased in the thermal power range, the Rayleigh number is increased along with the increase of the temperature of the wall surface of the cylinder, the hot air rises more severely under the action of buoyancy and forms stronger disturbance on the top of the cylinder, so that the convective heat transfer at the top is gradually increased, and the temperature difference between the top and the bottom of the cylinder is gradually reduced;
(3) When the thermal power is larger (Q is not less than 414.33W), the temperature difference between the top and the bottom of the cylinder increases continuously with the increase of the thermal power (the temperature difference increases from 3.67K to 6.61K). On one hand, the temperature of the wall surface of the cylinder is continuously increased along with the further increase of the thermal power, and a thicker thermal boundary layer is formed at the top of the cylinder after the hot air floats upwards, so that the heat convection between the wall surface of the top and the air is inhibited; on the other hand, although the temperature difference between the cylinder wall surface temperature and the ambient air is further increased, the kinetic viscosity of the air gradually increases with the increase in temperature, and the density of the air gradually decreases with the increase in temperature, which results in a significant increase in the kinematic viscosity of the hot air. According to Gr, pr and Ra of the Gradevil number of the air at the wall surface of the cylinder, the Rayleigh number is gradually reduced along with the increase of the air temperature in the thermal power range, so that the intensity of the plume and the convective heat exchange at the top of the cylinder is weakened, and the temperature difference between the top and the bottom of the cylinder is further increased.
Furthermore, as can be seen from fig. 5 (a) - (d), as the DBD discharge thermal power increases, the labeled difference of the thermocouple measurement temperatures at the same angular position of the cylindrical wall surface and the standard deviation of the temperatures at different angular positions
Gradually decreases in average value. This is because as the discharge voltage of the DBD reactor increases, the discharge uniformity within the cylinder is improved, andthe collision among the particles in the discharge area is more violent, so that the heating action of the air in the cylinder and the heat transfer of the air to the wall surface of the cylinder are more uniform, and the temperature distribution of the wall surface at the same angular position is more uniform. Comparing the experiment results of Shen et al about natural convection and radiation heat exchange in the cylindrical cavity, it can be found that when an electric heating coil is used as a heat source, the standard deviation of the surface temperature of the cavity is generally about 3K, while when a wire-cylinder type DBD reactor is used as a heat source in the experiment, the average standard deviation of the temperature of the cylindrical wall is at most 2.71K, when the thermal power reaches 99.04W,
already below 2K and the value decreases further with increasing thermal power. Therefore, the DBD reactor can ensure uniformity of heat flux density on the wall surface of the cylinder more than the electric heater, and can avoid experimental errors due to concentricity of installation of the heater, thereby being more suitable as a heat source for heating gas in the pipe from the viewpoint of experimental accuracy. In addition, in order to further improve the precision of the experimental result, when the DBD reactor is used as a heat source to carry out related research under different application scenes, the reactor is operated under higher discharge voltage and thermal power, and the accuracy of the experimental result is favorably improved.
S43: and comparing and analyzing the Knudel number result obtained by the experiment with a currently common empirical formula, verifying the effectiveness of the experimental system and the experimental method, and obtaining a new Knudel number correlation in the Rayleigh number range researched by the experiment through data fitting.
Preferably, the knossel number obtained in the experiment is compared with the existing correlation by combining the calculated results of the rayleigh number and the knossel number of the natural convection heat transfer of the cylinder wall surface with the temperature distribution under different thermal powers, as shown in fig. 6, wherein the existing correlation is as follows:
Nu＝0.456Ra 0.25
Nu＝0.474Ra 0.25 Pr 0.047
Nu＝0.424Ra 0.25
Nu＝0.47Ra 0.25
it can be seen from fig. 6 that the resulting knoevenagel data calculated by the correlation lie within the 20% deviation band of the experimental knoevenagel number. Therefore, the Knudel number data obtained by the experiment has higher accuracy, and the experimental method adopting the DBD reactor as the heat source in the cylinder is feasible.
The knoevenagel numbers obtained in this experiment were fitted to a correlation with Ra as the independent variable and data fitting was performed in the most common form Nu = aRab. Accordingly, the relationship between Nu and Ra under the present experimental conditions was obtained by linear fitting, as shown in fig. 7. It can be seen that the coefficient of determination R2 between the experimental data points and the fitted straight line is 0.9048, which indicates that the correlation between the two is good.
From the linear fit in FIG. 7, the Knudsen number correlation under the present experimental conditions can be expressed as
Nu＝0.338Ra 0.2788
This correlation, although applicable only to a small range of Ra values (1.8 × 105 ≦ Ra ≦ 3.5 × 105), was obtained for the first time by an experimental method using the DBD discharge proposed herein as a heat source, taking into account the nonuniformity of the temperature distribution of the cylindrical wall surface in the circumferential direction, and verified to have higher accuracy by comparison with the correlation obtained by other researchers. Therefore, the Knoop number correlation under the experimental condition can provide reference for relevant experimental research and numerical simulation research.
S5: and drawing a conclusion.
In order to solve the problem that the accuracy of a test result is low due to concentricity and heat source stability when a conventional electric heating mode is used as a heat source to measure the natural convection heat transfer coefficient of the outer wall surface of the cylinder in air, the invention considers the influence of radiation heat transfer of the wall surface of the cylinder, researches a correlation formula which adopts a linear-cylinder DBD reactor as a heat source in the cylinder, adopts a calorimetry to calculate the thermal power of the DBD reactor, and obtains the Knudel number within a certain Ra number range through the temperature distribution calculation of the outer wall surface of the cylinder. The main conclusions obtained by the invention are as follows:
(1) The DBD reactor is feasible as a heat source for heating the gas in the cylinder. By changing the discharge voltage of the DBD reactor, the heating power of the gas in the cylinder can be effectively adjusted, and an exponential corresponding relation exists between the heating power and the peak-to-peak voltage.
(2) The temperature distribution of the cylinder wall surface shows a trend of gradually descending from the top to the bottom, and along with the increase of the thermal power and the wall surface temperature, the temperature difference between the top and the bottom of the pipe wall shows a trend of increasing, decreasing and increasing under the influence of the temperature difference between the wall surface and air and convection disturbance.
(3) Compared with the conventional mode of installing an electric heater in a cylinder, the mode of adopting the DBD reactor as a heat source can obtain more uniform tube wall temperature, and the average standard deviation of the wall surface temperature at different angular positions is within 3K. Further, as the thermal power increases, the standard deviation of the wall surface temperature gradually decreases.
(4) The deviation of the Knudel number obtained through experiments and the data obtained through the existing correlation calculation is generally within a 20% deviation band, the average deviation of the Knudel number is mostly less than 10%, and the result of experimental tests is verified to have higher accuracy.
The invention verifies the feasibility and effectiveness of measuring the natural convection heat transfer coefficient of the wall surface of the pipeline by using the DBD reactor as a heat source.
The foregoing is merely exemplary and illustrative of the principles of the present invention and various modifications, additions and substitutions of the specific embodiments described herein may be made by those skilled in the art without departing from the principles of the present invention or exceeding the scope of the claims set forth herein.
Claims (8)
1. A DBD discharge device-based method for testing, analyzing and evaluating natural convection heat transfer coefficients of the surface of a circular tube is characterized by comprising the following steps: the method comprises the following steps:
s1: the method for measuring the thermal power transferred from the DBD reactor to the gas flow by adopting a calorimetry method specifically comprises the following steps:
s11: the volume flow O of air at the inlet is adjusted to 200L/min by adjusting the power-on frequency of the fan and the gas control valve]And calculating the airflow density rho according to the airflow temperature at the inlet of the cylinder in And then through the formula of calculation of the mass flow rate of the air flow
Obtaining aeration flow mass flow rate
S12: when the discharge of the reactor reaches the stable state, the temperature at the inlet and the outlet of the reactor is basically kept unchanged, and the thermal power P of the DBD reactor is obtained by calculating through a thermal power calculation formula of the DBD reactor a ；
S2: analyzing the average convective heat transfer coefficient, the radiant heat transfer coefficient and the Knudel number of the cylinder wall surface by adopting the average temperature of all temperature measuring points of the cylinder wall surface;
s3: analyzing uncertainty;
s4: testing results and analyzing;
s5: and drawing a conclusion.
2. The DBD discharge device-based method for testing, analyzing and evaluating the natural convection heat transfer coefficient of the surface of the circular tube according to claim 1, wherein the DBD discharge device comprises: the heat power calculation formula of the DBD reactor is as follows
In the formula, C p，in And C p，out The constant pressure specific heat, T, of the air at the inlet and outlet of the reactor, respectively in And T out The temperatures of the gas streams at the inlet and outlet of the reactor, respectively.
3. The DBD discharge device-based method for testing, analyzing and evaluating the natural convection heat transfer coefficient of the surface of the circular tube according to claim 2, wherein: the specific implementation method corresponding to the step S2 is as follows:
s21: setting 8 angular positions on the cylindrical wall surface, distributing 7 temperature measuring points in an axial distribution area at any angular position of the cylindrical wall surface, and collecting the temperature of each temperature measuring point;
s22: carrying out mean value processing on the temperatures of all temperature measuring points corresponding to all angular positions of the cylindrical wall surface to obtain the average temperature of any angular position theta of the cylindrical wall surface
T θ，i Expressing the temperature of the ith temperature measuring point at any angular position theta of the wall surface of the cylinder, wherein i is the number of the temperature measuring points, and i =1,2, ·,7;
s23: calculating the average temperature of the cylindrical wall surface based on the average temperature corresponding to each angular position of the cylindrical wall surface
The expression is
S24: detecting external ambient temperature T air And is based on
And T air Calculating the radiant heat exchange quantity Q in unit time r Wherein
Wherein ε represents the blackness of the cylinder wall surface, C 0 Is the absolute black body emissivity coefficient, d out Expressed as the outer diameter of the cylinder, L 0 Expressed as the length of the cylinder, and pi as the circumference ratio;
s25: calculating the radiation heat exchange coefficient of the cylinder wall surface, wherein the expression is
In the formula
S26: calculating the convective heat transfer coefficient h of the cylinder wall surface ec The expression is
In the formula Q c Expressed as convective heat transfer per unit time of the cylindrical wall surface, and Q c ＝Q-Q r Q represents the total heat exchange amount between the outer wall surface of the inner cylinder and the environment per unit time, and Q = P a ；
S27: calculating the average Nu of the cylindrical wall surface, wherein the expression is Nu = h ec d 0 Lambda, wherein lambda is expressed as the thermal conductivity;
s28: calculating the Grataffet number Gr, the Prandtl number Pr and the Rayleigh number Ra of the air at the wall surface of the cylinder, wherein Gr = g alpha delta Td out 3 /v 2 ，
Ra = Gr. Pr, where g is gravitational acceleration, ν is kinematic viscosity, α is expansion coefficient, and C p Expressed as specific heat and μ as viscosity coefficient.
4. The DBD discharge device-based circular tube surface natural convection heat transfer coefficient test analysis and evaluation method of claim 3, wherein the method comprises the following steps: the specific implementation method corresponding to the step S3 is as follows:
s31: analyzing mass flow rate of air
Uncertainty of (2)
The expression is
Wherein δ T in Expressed as the temperature uncertainty at the reactor inlet, δ O as the volumetric flow uncertainty of the air at the inlet;
s32: analysing the uncertainty δ P of the thermal power Pa of the discharge a The expression is
Wherein δ C p，in Expressed as the uncertainty of the specific heat at constant pressure of the air at the inlet of the reactor, deltaC p，out Expressed as the uncertainty of the specific heat at constant pressure of the air at the outlet of the reactor, deltaT out Expressed as the temperature uncertainty at the reactor outlet, a, b, c, d are constants;
s33: analyzing uncertainty delta h of radiation heat transfer coefficient of cylinder wall surface er The expression is
Wherein δ Q r Expressed as the uncertainty of the radiant heat exchange per unit time, and
δd out expressed as the uncertainty, δ L, of the cylinder outer diameter 0 Expressed as an uncertainty in the length of the cylinder,
expressed as the uncertainty of the mean temperature of the cylinder wall, δ T air Expressed as the uncertainty of the external ambient temperature;
s34: analyzing uncertainty delta h of convective heat transfer coefficient of cylinder wall ec The expression is
Wherein δ Q c Expressed as the uncertainty of the convective heat transfer quantity per unit time, and
δ Q represents the uncertainty of the total heat exchange between the outer wall surface of the inner cylinder and the environment per unit time;
s35: analyzing uncertainty delta Nu of average Knudel number of cylinder wall surface, its expression is
δ λ represents the uncertainty in thermal conductivity.
5. The DBD discharge device-based circular tube surface natural convection heat transfer coefficient test analysis and evaluation method of claim 4, wherein the method comprises the following steps: in the step S32
6. The DBD discharge device-based circular tube surface natural convection heat transfer coefficient test analysis and evaluation method of claim 4, wherein the method comprises the following steps: in the step S32
7. The DBD discharge device-based method for testing, analyzing and evaluating the natural convection heat transfer coefficient of the surface of the circular tube according to claim 1, wherein the DBD discharge device comprises: the step S4 specifically includes:
s41: analyzing a relationship between a thermal efficiency of the DBD reactor discharge and a discharge voltage to determine whether the DBD discharge can provide a sufficient heat source;
s42: analyzing the influence of different thermal powers on the temperature distribution uniformity of the outer wall surface of the cylinder to evaluate the feasibility of the DBD reactor as a heat source in the cylinder;
s43: and comparing and analyzing the obtained Nossel number result with the existing correlation, verifying the effectiveness of the experimental system and the experimental method, and obtaining a new Nossel number correlation in the Rayleigh number range through data fitting.
8. The DBD discharge device-based method for testing, analyzing and evaluating the natural convection heat transfer coefficient of the surface of the circular tube according to claim 1, wherein: the conclusion drawn in S5 is as follows:
(1) DBD reactors are feasible as a heat source for heating gas in a cylinder;
(2) The temperature distribution of the wall surface of the cylinder shows a trend of gradually decreasing from the top to the bottom, and along with the increase of the thermal power and the temperature of the wall surface, the temperature difference between the top and the bottom of the pipe wall shows a trend of increasing, decreasing and increasing under the influence of the temperature difference between the wall surface and air and convection disturbance;
(3) Compared with the conventional mode of installing an electric heater in a cylinder, the mode of adopting the DBD reactor as a heat source can obtain more uniform tube wall temperature, and the average standard deviation of the wall surface temperatures at different angular positions is within 3K;
(4) The deviation of the Knudel number obtained through experiments and the data obtained through the existing correlation calculation is generally within a 20% deviation band, the average deviation of the Knudel number is mostly less than 10%, and the accuracy of experimental test results is verified.
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| CN116256390A (en) * | 2023-04-19 | 2023-06-13 | 中国人民解放军陆军装甲兵学院 | Experimental test method for forced convection heat exchange parameters of air flow in cylinder under condition of thermal physical property parameters |
| CN116519738A (en) * | 2023-03-31 | 2023-08-01 | 中国人民解放军陆军装甲兵学院 | Method for testing and analyzing heat transfer parameters of plasma jet in circular tube under magnetic-flow-thermal multi-field coupling |
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| QIJIN ZHAO等: "Experimental investigation of the natural convection heat transfer characteristics of cylinder walls with a DBD actuator as the heat source", CASE STUDIES IN THERMAL ENGINEERING, pages 3 - 11 * |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN116519738A (en) * | 2023-03-31 | 2023-08-01 | 中国人民解放军陆军装甲兵学院 | Method for testing and analyzing heat transfer parameters of plasma jet in circular tube under magnetic-flow-thermal multi-field coupling |
| CN116256390A (en) * | 2023-04-19 | 2023-06-13 | 中国人民解放军陆军装甲兵学院 | Experimental test method for forced convection heat exchange parameters of air flow in cylinder under condition of thermal physical property parameters |
| CN116256390B (en) * | 2023-04-19 | 2024-01-23 | 中国人民解放军陆军装甲兵学院 | Experimental test method for forced convection heat exchange parameters of air flow in cylinder under condition of thermal physical property parameters |
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